Push-pull linear amplifiers are commonly used as audio amplifiers. In such amplifiers a great deal of power is usually dissipated across the output transistors. This results in a low efficiency, typically 35 per cent, which is exhibited by most products presently on the market. The large amount of power dissipated (wasted) by the output transistors requires large power supplies, transistors and heat sinks. Such amplifiers are therefore large and require significant ventilation.
Attempts have been made to improve the efficiency of such amplifiers. For example, as disclosed in an article by Harushige Nakagaki et.al. in J. Audio Eng. Soc., Volume 31, No.6, June, 1983 at page 430, the DC voltage supply for an amplifier can be controlled so that it changes in accordance with the input signal. This greatly reduces the power dissipation in the output transistors. However the efficiency of the Nakagaki amplifier is still only 50 per cent, which means that for a 200 watt (output) amplifier, 200 watts must still be dissipated as heat.
Other methods have been used in an attempt to produce more efficient amplifiers. For example switch mode amplifiers have been used, as will be described in more detail presently. These amplifiers are much more efficient than ordinary push-pull linear amplifiers, but they are complex and subject to distortion. It is difficult to achieve high quality audio output signals at a reasonable cost with present switch mode amplifiers.
Bridge audio amplifiers have also been used, as described in a paper by Takahashi et.al., described in J. Audio Eng. Sos., Volume 32, No. 6, June, 1984. Such amplifiers can produce very low distortion, but again they suffer from low efficiency and the need to dissipate substantial power.